Classification of Stress Action. The quantities in this table are given in tons, because the numbers are thus more easily remembered, and because it is the Engineer's language. Fig. 350 shews them diagrammatically. (See Appendicesi pp. 840 and 1075 for further materials.) The Proportioning of Structures and Machine Parts by Calculation.—The equality of action and reaction is the starting point in constructive calculation. Whether the load be applied directly or through a lever arm, the external forces must balance the internal stresses, and we have for the two cases: (Direct action). Total load = Total stresses. (Lever action). Moment of load = Moment of stresses,* which are our general strength equations. Classification of Stress Action. — Practical cases of simple or compound stress may be arranged under ten heads:— * A moment = force x lever arm.